Remote Sensing of Hydrocarbon Microseepage

Remote sensing can help professionals in the oil and gas industry discover new areas for exploration. One such application is identifying areas of hydrocarbon microseepage.

Microseeps are invisible traces of light hydrocarbons (primarily C1 – C5 substances) that seep to the surface from an oil or gas reservoir. When hydrocarbons migrate upward and reach exposed air at the surface, several manifestations occur at or near the surface that we can monitor with remote sensing. While remote sensing cannot directly detect the presence of microseeps, it can exploit potential areas of microseepage that could indicate an underlying reservoir.

Researchers have studied this for several decades.When I did my graduate research on this subject 18 years ago, I was limited to the use of Landsat TM imagery. Today we can use super-spectral shortwave-infrared (SWIR) sensors and LiDAR to explore potential petroleum reserves. In this article, I will give a quick introduction and some examples from Landsat and ASTER imagery.

Background

As light hydrocarbon gases move toward the surface and come in contact with the atmosphere, they are oxidized away through a process called bacterial oxidation. This increases the acidity (pH) of the near-surface soils. Soil samples are often needed to confirm these changes to the soil geochemistry, but we can also look for large-scale manifestations through remote sensing. Here are some common indicators of hydrocarbon-induced alteration (Yang, Van der Meer, and Zhang, 2000):

Bleaching of red beds: Sandstone formations normally have a reddish color if they contain iron oxide (hematite). When hydrocarbons migrate upward, they dissolve the iron oxide and bleach the sandstone. These areas will show a reduction of ferric iron with an increase in pyrite, siderite, uraninite, elemental sulfur, and some magnetic iron oxides and iron sulphides.

Clay mineral alteration: Feldspars and mixed-layer clays such as illite and smectite will convert to kaolinite. Past research has shown that some sandstones contain three to five times more kaolinite than unbleached sandstone. Kaolinite-rich sandstone shows higher values in a clay ratio image (for example, Landsat TM bands 5/7).

Increased carbonates: This is a by-product of hydrocarbon oxidation.

Stressed vegetation: Vegetation occupying the acidic soils will take in different transition elements through their roots. The vegetation will often show abnormal spectral signatures as a result of the changes in soil geochemistry.

To fully analyze areas of potential microseepage, we need more than remote sensing imagery. Field studies are required as well as a thorough knowledge of the underlying geology and fault structures. This article focuses on the remote sensing aspect.

Let's look at Kern County, California as an example. The Midway Valley west of Bakersfield contains several massive oil and gas fields. One of these is the Midway-Sunset oil field, which produced 29.3 million barrels of oil in 2014, making it the third largest oil-producing field in the U.S. The Elk Hills field is the largest gas-producing field in California, yielding 80.8 billion cubic feet in 2014.

Study area showing locations of major oil/gas fields. Images courtesy of Google.

A view of the McKittrick and Elk Hills fields, looking north. Image courtesy of Google.

While this region already has been heavily explored, we can still use it to understand how a hydrocarbon-rich area looks in imagery.

ASTER is a commonly used sensor for detecting minerals and alteration zones because of its range of bands that cover the visible through SWIR wavelengths. The following image was derived from an ASTER scene in 2007 over the area of interest. Looking for alteration zones is a first step in identifying areas of potential microseepage.

ASTER band ratio image created in ENVI 5.3 that highlights alteration zones, alunite, kaolinite, and pyrophyllite; while suppressing areas of ferric iron. After a "Haze" color table is applied, the alteration zones are shown in yellow. Agricultural fields and bright playas were masked out prior to creating the band ratio so that they did not bias the results.

The next image shows a Landsat-8 scene from 2013 (courtesy of the U.S. Geological Survey). I created a color composite that highlights carbonates in a pink color. You can really the increased carbonates in the valley where the oil and gas fields reside.

ASTER and Landsat are good for highlighting areas of mineral alteration due to hydrocarbons over large geographic areas; however, they can only provide a starting point for further exploration. WorldView-3, for example, has a finer spatial resolution and a wider range of SWIR bands that can help refine our study of hydrocarbons. We can also look at areas of vegetation stress using hyperspectral sensors. I will show some examples of these in future articles.